King Mosiah was the seer who used the Urim and Thummim to translate the record of the Jaredites as contained on 24 plates of gold (Mosiah 28:11-13). Moroni is intimating that Mosiah translated the story of Mahonri's vision and kept it from going abroad among the Nephites according to the command of the Lord. Moroni could as well have said, for this cause did Mosiah keep them secret.
We are left to wonder why the Lord did not want this vision made public prior to his first advent. The reason for this is not apparent except that the experience was so sacred and holy as to be inappropriate for all but the most righteous. Even what we have now of Moses' great vision is coupled with the caution, Show them not unto any except them that believe. Even so. Amen. (Moses 1:42).